State highway officials plan to start early next year with a six-month study into the feasibility of an alternative route around an 8-acre sinkhole in northern Assumption Parish.
Though carrying just 6,000 vehicles per day, La. 70 cuts through isolated swamp and provides a key connector between La. 1 along Bayou Lafourche and parts west and south, including Pierre Part and Morgan City.
The growing sinkhole is 1,200 feet south of La. 70 South between Grand Bayou and Bayou Corne. Current estimates have the sinkhole, believed to have been caused by a failed Texas Brine Co. LLC cavern in the Napoleonville Dome, about 640 feet across from north to south and 690 feet across from east to west.
The highway study is the initial step the Louisiana Department of Transportation and Development takes to consider a proposed highway before basic design begins, DOTD officials said.
DOTD spokesman Dustin Annison said the study will determine if “the route could be built in the affected area in the event that La. 70 is compromised.”
“The study is the first step in the project development process and will help identify possible roadway alignments and cost,” Annison said.
He said DOTD is in the process of defining the scope of work and expects to finish midyear.
Though no cost estimate is available, Annison said that a new two-lane rural highways costs $4 million to $6 million per mile to build, including right of way and wetlands mitigation costs.
The announcement comes as community concerns linger about the long-term viability of the highway despite authorities’ assurances that the sinkhole does not appear to be threatening the future of La. 70.
The sinkhole was found early Aug. 3 on property that Texas Brine leases from Los Angeles-based Occidental Petroleum Corp., swallowing up forested swamps on the western edge of the 40-acre site.
The discovery prompted an evacuation of about 150 homes in the area but also temporarily closed a four- mile stretch of La. 70 between Bayou Corne and Pierre Part when the sinkhole bent a gas pipeline a day after the sinkhole was found.
The closure was the third since 2003 in the area. The closures were noted nearly two weeks later when the Assumption Parish Police Jury adopted a resolution Aug. 22 calling for DOTD to look at a new “bypass” north of La. 70, linking it with the intersection of La. 69 and La. 996.
The two earlier closures between La. 1 and La. 69 occurred in 2003 near Grand Bayou because of a gas leak at a well and in 2010 when a well blew out near Paincourtville.
The sinkhole, however, is just west of La. 69, and any closure would cut off Bayou Corne and Pierre Part from nearby highways north or south that would provide a direct route toward La. 1 and Napoleonville — instead sending drivers through Morgan City or White Castle.
Police Juror Henry Dupre, who represents the Bayou Corne area, said concerns about another shut down of La. 70 remain on people’s minds, though the worry may have eased off some.
“You know that’s the main corridor for four parishes,” Dupre said.
Dupre said Assumption High School is in Napoleonville and students Pierre Part take a bus on La. 70 to the school.
Jamie Ponville, owner of Pierre Part-based Jamie Ponville Trucking LLC, said people can see the sinkhole and the oil retardant booms surrounding the sinkhole from La. 70.
Ponville, 36, said losing access to La. 70 would be “very rough businesswise.”
“Most everybody I am speaking with are concerned about the highway,” Ponville said. “It’s our only access to the rest of the parish. If Highway 70 is shut down, we are shut down from the rest of the parish.”
The brine-filled slurry hole has had expected edge collapses, causing the sinkhole to grow from an area of about two football fields to one now covering about six.
Scientists working for the Louisiana Office of Conservation and Shaw Environmental and Infrastructure have estimated the sinkhole could ultimately grow to 1,500 feet in diameter.
That’s an area covering more than 30 football fields, or about 40 acres.
Patrick Courreges, Louisiana Department of Natural Resources spokesman, which includes the Office of Conservation, said those scientists believe the “border of the maximum expansion of the sinkhole is anticipated to be well short of reaching Highway 70 and the Bayou Corne community.”
DOTD’s Annison said the agency does weekly monitoring of La. 70’s elevation at 20 locations and at the three bridges on the salt dome and has not found the highway’s elevation changed since the sinkhole appeared.
DOTD also inspects the bridges at Bayou Corne, Grand Bayou and Bayou Choupique weekly, including the supports under the bridge deck, and has not detecting any movement.
“DOTD continues to monitor the roads in the area and, at this time, has no concerns related to the integrity of its state roads, specifically La. 70 in Assumption Parish,” he said.
Annual subsidence surveys of the salt dome show the dome has subsided between 1995 and 2012, and La. 70 crosses over the dome east of the sinkhole. The survey shows subsidence has been most pronounced near the center of the dome, about an inch a year.
The center of the dome is south La. 70 and east of Grand Bayou and the sinkhole.
Subsidence on the perimeter of the dome has averaged about a quarter-inch annually, according to the report dated from August.
The report’s author, consultant Joe Ratigan, declined comment. PB Storage did not return messages for comment.
Stephen Bauer, a geologist with Sandia National Laboratories in Albuquerque, N.M., which has done extensive research on salt domes, reviewed the report and said it shows fairly typical subsidence for a salt dome with caverns inside it.
Bauer said subsidence occurs because material has been removed from underground and salt undergoes “creep” that closes in the hollowed out caverns.
“It’s exactly what one might expect,” Bauer said. “The salt goes down and has moved down in response to where the salt has been removed at depth.”
He said the sinkhole would not affect this subsidence, but dome subsidence would affect things at the surface, like roads.
DOTD’s Annison said La. 70 monitoring began shortly after the sinkhole emerged and does not take into account “subsidence that has been occurring in this area over the past couple of decades.”
Some residents welcomed the look at an alternative route, fearing the sinkhole could affect the highway.
“Look what (the sinkhole’s) done so far,” Viki Arnold, 27, said. “Have they predicted anything it was gonna do? No. That thing’s going to do whatever ... it wants. I don’t care what DNR or anybody else says.”
Viki Arnold was with her husband, Richard, 57, at the Gator Corner gas station lunch room Friday, a business that would be cut off from Bayou Corne if La. 70 were shut near the sinkhole. The Arnolds now live in Paincourtville after leaving Bayou Corne.
“They messed with Mother Nature, and Mother Nature’s paying them back,” Richard Arnold said.
“Yep,” Viki said. “And she don’t play fair.”
For information on Napoleonville Dome subsidence, visit http://dnr.louisiana.gov/assets/oc/bc_all_updates/12.napoleonville.subreport.pdf.